1. Field
Certain aspects of the present disclosure generally relate to wireless communications and, more particularly, to methods and apparatus for medium reservation in the case of multi-user transmissions.
2. Background
In order to address the issue of increasing bandwidth requirements that are demanded for wireless communications systems, different schemes are being developed to allow multiple user terminals to communicate with a single access point by sharing the channel resources while achieving high data throughputs. Multiple Input Multiple Output (MIMO) technology represents one such approach that has recently emerged as a popular technique for the next generation communication systems. MIMO technology has been adopted in several emerging wireless communications standards such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. The IEEE 802.11 denotes a set of Wireless Local Area Network (WLAN) air interface standards developed by the IEEE 802.11 committee for short-range communications (e.g., tens of meters to a few hundred meters).
The IEEE 802.11 WLAN standards body established specifications for transmissions based on the very high throughput (VHT) approach using a carrier frequency of 5 GHz (i.e., the IEEE 802.11ac specification), or using a carrier frequency of 60 GHz (i.e., the IEEE 802.11ad specification) targeting aggregate throughputs larger than 1 Gigabits per second. One of the enabling technologies for the VHT 5 GHz specification is a wider channel bandwidth, which bonds two 40 MHz channels for 80 MHz bandwidth therefore doubling the physical layer (PHY) data rate with negligible increase in cost compared to the IEEE 802.11n standard.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS ≦min {NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
In IEEE 802.11 networks, transmissions can occur by following a random medium access mechanism called Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Transmissions from one node can happen concurrently with transmissions from other nodes in the network, and this situation can be referred to as a collision. The CSMA/CA mechanism tries to avoid collisions by having nodes (i.e., user stations (STAs)) sense the medium before starting a transmission in order to ensure that no other STA is already transmitting. In some configurations, not all the STAs are able to hear each other, and the sensing mechanism can fail. This can be referred to as the hidden node scenario. In order to limit the latter case, the concept of Network Allocation Vector (NAV) is present in the IEEE 802.11 standard, wherein NAV information may comprise an indication of time for which the medium is going to be busy. This indication may be relied to the hidden nodes by using appropriate messages.
The IEEE 802.11 standard specifies the use of Request-to-send/Clear-to-send (RTS/CTS) messages to provide the NAV information to hidden node(s), and hence provide protection for the immediately following transmission. The RTS/CTS mechanism can be also useful in lowering the overhead caused by collisions. If an RTS message is transmitted before the data and a collision happens, then the CTS message will be missing, which allows to identify the collision event. Also the RTS message is typically much shorter message than data, and hence the time taken by the collision is short.
The RTS/CTS mechanism can also allow detecting the NAV being set by neighboring networks, which may be hidden to the transmitter. If a CTS message is not received, the reason can be that the NAV for the RTS receiver was set, preventing the RTS receiver to reply with a CTS message.
A Multi-User MIMO (MU-MIMO) transmission in IEEE 802.11 networks may comprise data destined to multiple STAs scheduled for simultaneous transmission. In this case, efficient protection of the MU-MIMO transmission is desired.